1.Field of the Invention
This invention relates to a novel apparatus for reducing stresses due to bending in threaded fasteners, particularly bolts and cap screws, used in connecting flanged connections. Such flanged connections have a wide variety of industrial uses including connecting structural members that will be subjected to critical external loads and in sealing high pressure vessels. The flanges used in such applications must be maintained in face to face contact to ensure proper operation. This face to face contact is maintained by the plurality of threaded fasteners that are tightened to a high percentage of their yield strength to ensure this contact is maintained. It is well known in the art that such flanges bend in an axial or meridional direction during this tightening procedure. This meridional bending is transmitted to the threaded fasteners and is known to contribute significantly to the stress in the threaded fastener and reduces the amount of tightening to which the threaded fastener can be safely subjected. This in turn reduces the face to face contact between the flanges and therefore the load to which the flanged connection can be subjected.
The present invention has particular applicability in the oil and gas industry where large diameter tubular members with flanged end connections, called xe2x80x9crisersxe2x80x9d or xe2x80x9criser jointsxe2x80x9d, are used. These risers are used in offshore drilling and production operations and extend from the wellhead at the ocean floor to a surface vessel. These risers have typically been manufactured in fifty and eighty foot lengths. During normal operation the flanged connection between adjacent riser joints must contain internal pressure along with enormous external loads due to environmental conditions. It is critical that these flanged connections, and the threaded fasteners holding them together, function flawlessly. These flanged connections and the threaded fasteners connecting them are the area to which the present invention most closely applies.
2. Description of Related Art
An apparatus for stretching threaded fasteners is shown in U.S. Pat. No. 3,749,362 (O""Connor et al.) The apparatus shown by O""Connor et al. uses an external fastener stretcher threaded onto a bolt to apply an axial load to the bolt wherein the bolt""s nut can then be tightened with minimal torque. Removal of the fastener stretcher allows the bolt and nut to retain the load induced in the bolt.
The current invention uses a radially tapered surface that is machined into the backface of the flange, adjacent to the bolt or stud holes, to compensate for the axial or meridional direction bending that the flange undergoes during fastener tightening. The machined radially tapered surface ensures that as the flanges bend the threaded fastener remains straight and thereby minimizes the bending stress transmitted to the fastener.
According to the present invention, in a first embodiment a radially tapered surface is machined into the backface of the upper flange adjacent each of the bolt holes. The lower flange has a plurality of complementary holes threaded to receive the bolt. As the bolt fastener is tightened to its predetermined value and the flange deflects, the head of the fastener remains vertical while the radially tapered surface moves downwardly. At maximum fastener torque and flange deflection, the machined radially tapered surface of the backface of the flange is in substantially face to face contact with the head of the fastener and perpendicular to the axis of the fastener.
In a second embodiment of the invention, an all thread stud with a single nut is used in place of the bolt of the first embodiment. A radially tapered surface is machined into the backface of the upper flange adjacent each of the stud holes. The lower flange has a plurality of complementary holes threaded to receive the all thread stud. As in the first embodiment, as the nut of the fastener is tightened to its predetermined value and the flange deflects, the head of the fastener remains vertical while the radially tapered surface moves downwardly. At maximum fastener torque and flange deflection, the machined radially tapered surface of the backface of the flange is in substantially face to face contact with the nut of the fastener and perpendicular to the axis of the fastener.
In a third embodiment of the invention, an all thread stud with a pair of nuts is used in place of the fasteners of the previous embodiments. A radially tapered surface is machined into the backface of the upper and lower flanges adjacent each of the stud holes. As in the previous embodiments, as the nuts of the fastener are tightened to their predetermined value and the flange deflects, the fastener remains vertical while the radially tapered surface moves downwardly. At maximum fastener torque and flange deflection, the machined radially tapered surfaces of the backface of the flanges are in substantially face to face contact with the nuts of the fasteners and perpendicular to the axis of the fastener.
A principal object of the present invention is to provide an apparatus to reduce the bending stress transmitted to threaded fasteners due to deflection of the flange.
Another object of the present invention is to provide an apparatus for increasing the fatigue life of threaded fasteners used in flanged connections.
A final object of the present invention is to provide an apparatus for reducing the bending stress in threaded fasteners without requiring the use of special fasteners.
These with other objects and advantages of the present invention are pointed out with specificness in the claims annexed hereto and form a part of this disclosure. A full and complete understanding of the invention may be had by reference to the accompanying drawings and description of the preferred embodiments.